The present invention relates to a power steering apparatus.
In a related electric power steering apparatus described in, for example, JP-A-2002-29431, torque T is calculated from a relative difference between steering angles in an input shaft of the input side and an output shaft of the output side from a torsion bar in a steering shaft. Then, when it is determined that the torque T is backward input torque based on each angular speed and vehicle speed of the input shaft and the output shaft, a sign (+, −) of the torque T is reversed and the torque T is set to backward assist torque, and a motor for steering assistance is driven in order to generate it. Incidentally, the case that the torque T becomes the backward input torque is, for example, the case that disturbance from a road surface is received in a steering direction of wheels in a steering holding state at the time of straight travel.
Thus, the backward input torque due to the disturbance is offset by driving force of the motor to prevent the disturbance from propagating to a handle.
However, in the related electric power steering apparatus as described above, unnatural steering feeling may be given to a driver by performing a backward assist operation in which the disturbance is offset regardless of driver's intention. Since the disturbance hardly propagates to the handle, it is difficult for the driver to accurately grasp a state of the road surface. Giving an extreme example, there is little difference between the case of running a snowy road with low frictional resistance of the road surface in a steering direction and the case of running a gravel road with high frictional resistance in the disturbance propagating to the driver through the handle. That is, information about the road surface through the handle does not propagate to the driver accurately.